For a long time, we thought avocadoes were good for nothing but ready-made guac and a decent California burger every now and then. But these little nutritional hand grenades were having an explosive impact on our diets for all that time. How so? They’re infused with a key nutrient for maintaining healthy weight: fat.

Wait…fat can help us maintain our weight? Fat doesn’t make us fat? In a word: exactly.

Fat is not something to avoid. For starters, it’s essential for normal growth and development. Dietary fat also provides energy, protects our organs, maintains cell membranes, and helps the body absorb and process nutrients. Even better, it helps the body burn fat, says nutritionist and owner of Nutritious Life meal system, Keri Glassman, RD, who recommends that about a third of any weight-loss plan’s calories come from dietary fat.

Before you grab a deep-fried hot dog, consider this: not all fatty foods are created equal. The foods you choose can mean the difference between a trim body and one plagued with obesity and disease, Glassman says. While a diet of stereotypically fatty foods like pizza, French fries, and hamburgers can contribute to weight gain and deterioration of health, the dietetic community is learning that the overall nutritional content of these foods—not their saturated fat—is what’s to blame. Sure, research from 50 years ago found that saturated fatty acids, a type of fat that’s “saturated” with hydrogen and typically solid at room temperature, raised LDL (bad) cholesterol levels.

But a reevaluation of that research has shown that they raise HDL (good) cholesterol just as much, if not more, protecting the body from unhealthy cholesterol levels and heart disease, says nutritionist and national spokesperson for the American Dietetic Association Tara Gidus, RD. “Instead of making any one thing in the diet a villain, we need to look at total caloric content as well as quality of food, what are we eating that is ‘good’ and helping our body’s immune system and cells to stay healthy.”

Most of the fat that you eat—especially if you want to lose weight—should come from unsaturated sources, both monounsaturated (MUFA) and polyunsaturated (PUFA), Glassman says. Why? These good-for-you foods (like fish, seeds, nuts, leafy vegetables, olive oil, and, of course, avocadoes) pack tons of nutrients. Besides removing LDL cholesterol from arteries and promoting a healthier heart, unsaturated fat can help you burn fat big time without cutting calories. A 2009 study in the British Journal of Nutrition, found that participants who consumed the most unsaturated fatty acids have lower body mass indexes and less abdominal fat than those who consumed the least. Why? The unsaturated folks ate higher-quality foods.

Not long ago, the low-fat/no-fat diet craze swept across the food landscape. Manufacturers marketed low-fat and no-fat everything, and consumers responded by chowing down. It’s healthy, right? Wrong. All wrong. Besides stripping our bodies of a much-needed nutrient, low- and no-fat diet movements have increased obesity rates. Why? It turns out that fat provides a big component to the foods we love: Taste. When food manufacturers removed fat from their foods, they had to load the foods with sugar and salt, which are nutrient-free, to increase flavor. For example, the second most prevalent ingredient Kraft Fat-Free Catalina salad dressing, for instance, is high fructose corn syrup, packing 7g of sugar per serving. And just one ounce of the saucy stuff packs 350mg of sodium—that’s 15% of your recommended daily value—and who eats just one “serving,” anyway?

And that’s just the start. Here are other crucial ways fat can help you slim down:

“Instead of making any one thing in the diet a villain, we need to look at total caloric content as well as quality of food, what are we eating that is ‘good’ and helping our body’s immune system and cells to stay healthy"

Tara Gidus, RD and national spokesperson for the American Dietetic Association

What to eat – and what to skip – when adding fat to your diet

*Unsaturated Fatty Acids: Consisting of both monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs), they are important for health. MUFAs are found in vegetable oils, nuts, seeds, olives, and avocadoes, while PUFAs are found in vegetable oils, fish, and seafood. Omega-3 and omega-6 fatty acids are two PUFAs that can only be obtained through diet and are called “essential fatty acids.” Adjust your diet accordingly.

*Saturated Fatty Acids: Found primarily in foods from animal sources such as meat and dairy products, like butter and cheese, they are usually solid at room temperature. Some vegetable oils such as coconut, palm kernel, and palm oil also contain saturated fat. Eat limited amounts as part of a healthy diet – and always try to consume healthier sources. For example, grass fed beef is a much better option than, say, popcorn popped in oil.

*Trans Fatty Acids: Chemically processed vegetable oils, they are semisolid at room temperature and are used in some margarines, fried foods, and processed foods to enhance flavor, texture, and shelf life. Also called “partially hydrogenated” oils, they should be avoided like the plague they are.

Fat Burns Fat

The body needs three macronutrients for energy: Carbohydrates, protein, and fat. A gram of fat packs more than twice the energy of a gram of the other two. “When you don’t have any fat in your diet its like you don’t have fuel to burn calories,” Glassman says. The body requires energy to keep its metabolism properly functioning, and a 2007 study published in the American Journal of Clinical Nutrition found that consuming fatty acids can boost metabolic health.

What’s more, “old” fat stored in the body’s peripheral tissues—around the belly, thighs, or butt (also called subcutaneous fat)—can’t be burned efficiently without “new” fat to help the process, according to researchers at Washington University School of Medicine in St. Louis. Dietary fat helps break down existing fat by activating PPAR-alpha and fat-burning pathways through the liver.

Think of mealtime like baseball spring training: young, hungry players (new fat) hit the field and show the general manger (the liver) that it’s time to send the old, worn-out players (subcutaneous fat) home. And away they go.

Fat Keeps You Full

Fat isn’t the easiest nutrient to digest, so it sticks around in the digestive system for more time than many other nutrients. MUFAs may also help stabilize blood sugar levels, according to Mayo Clinic. That means you feel full longer, and you won’t feel the stomach-growling urge to raid the refrigerator after mealtime.

In fact, diets with high amounts of omega-3 fatty acids, a type of MUFA that the body can only acquire through food, create a greater sense of fullness both immediately following and two hours after dinner than do meals with low levels of the fatty acids, according to a 2008 study from University of Navarra in Pamplona, Spain. It’s no surprise that dieters who consume moderate levels of fat are more likely to stick with their eating plans than dieters who consume low levels of fat.

The result? More weight lost.

Fat Makes You Happy

Everyone says that dieting, not to put too fine a point on it, stinks. Eating yummy foods makes you happy, and it turns out low-fat versions just don’t do the trick for one surprising reason: We can taste the fat—not just the salt, sugar, and other goodies in food.

Recent research from Purdue University shows that our taste buds can detect fat in food, which helps explain why low-fat foods don’t curb our fat cravings. According to the research, fat may be an entirely different basic taste than what we’ve long considered the four mainstays: sweet, salty, sour, and bitter. On an even happier fat note, omega-3 fatty acids can boost serotonin levels in the brain, helping to improve mood, increase motivation, and keep you from devouring a large pizza like it’s your job. 3.5% of women and 2% of men have suffered from diagnosed binge-eating disorders, while millions more people are occasional emotional eaters, according to the National Institutes of Mental Health.

Fat Builds Muscle

“Eating good fats along with an effective exercise program can increase muscle,” says trainer and owner of Results Fitness, Rachel Cosgrove, CSCS, who notes that increasing muscle mass is vital to increasing metabolism and burning calories both in and out of the gym. In a 2011 study published in Clinical Science, researchers examined the effects of eight weeks of PUFA supplementation in adults ages 25 to 45 and found that the fat increases protein concentration and the size of muscular cells in the body. Previous studies have found that omega-3 fatty acids stimulate muscle protein synthesis in older adults and can mediate muscle mass loss due to aging.

Fat Makes Food Better For You

Many nutrients including vitamins A, D, E, and K are fat-soluble, meaning that the body can’t absorb them without fat. If your body isn’t absorbing nutrients properly, that can lead to vitamin deficiencies and bring on dry skin, blindness, brittle bones, muscle pains, and abnormal blood clotting, according to Gidus.

These vitamins are also key to maintaining energy, focus, and muscle health, all of which contribute to a healthy weight. Vitamin E, for example is a powerful antioxidant and helps maintain your metabolism, while the body’s levels of vitamin D predicts its ability to lose fat, especially in the abdominal region, according to a clinical trial from the University of Minnesota Medical School. So while you can pile your salad high with nutrient-rich spinach, tomatoes, and carrots, you really need to thank the olive oil for sending the salad’s vitamins your way.